This will not currently create valid NetBSD binaries because NetBSD requires
an ELF note section to run, otherwise the kernel will throw ENOEXEC. I was
unable to determine an elegant way to add the section, so I am submitting
what I have.

References:
http://www.netbsd.org/docs/kernel/elf-notes.html
http://mail-index.netbsd.org/netbsd-bugs/2001/08/03/0012.html

R=rsc
CC=golang-dev
https://golang.org/cl/5472049